An exhaust gas purification system using a ceramic catalytic converter as means for removing carbon monoxide (CO), hydrocarbon (HC), nitrogen oxides (NOx), etc, contained in an exhaust gas of a car engine from the exhaust gas is well known. The ceramic catalytic converter basically accommodates a honeycomb-shaped ceramic catalyst support (also called “catalyst element”) in a metal casing or housing.
Various kinds of ceramic catalytic converters are available as is well known in the art. Generally, the ceramic catalytic converters employ a construction in which a clearance or gap between a catalyst support disposed in a casing and the casing is filled with a heat insulating/mounting material typically comprising the combination of an inorganic fiber with an organic fiber and/or a generally liquid or paste-like organic binder. As a result, the heat insulating/mounting material filling the clearance or gap retains the catalyst support and can prevent a mechanical shock resulting from impact, vibration, and so forth, from accidentally acting on the catalyst support. In the catalytic converters having such a construction, breaking and movement of the catalyst support do not occur and an intended operation can be accomplished for an extended period. Incidentally, when such a heat insulating/mounting material is in a form that functions to retain or mount the catalyst support, it is generally called a “catalyst support mounting system”, too.
Incidentally, when the catalyst support is loaded into the casing, a push-in system that winds the catalyst support mounting system around an outer circumference of the catalyst support, integrates them together and then pushes the integrated body into a cylindrical casing under pressure has generally been employed. Various types of catalyst support mounting systems have been proposed at present in order to improve the ability to assemble the catalyst support in a catalytic converter using the push-in system (called “canning”), to improve a cushion property (resiliency) of the catalyst support mounting system and to prevent scattering of inorganic fiber used in the catalyst support mounting system in air. As shown in FIG. 1, for example, in a catalytic converter 35 including a catalyst support 33, a metal shell (casing) 32 covering an outer periphery of the catalyst support 33 and a retaining seal member 31 interposed between the catalyst support 33 and the casing 32, a catalyst support mounting system which comprises an inorganic fiber mat subjected to needle punch treatment into a density of 50 to 3,000 pcs/100 cm2, has an organic content of 0 to 2 wt % or below and generates a surface pressure of 5 to 500 kPa when heated to a temperature between 300 to 1,000° C. at a packing density of 0.15 to 0.45 g/cm3 has been proposed. See Japanese Unexamined Patent Publication (Kokai) No. 2001-259438. Here, the retaining seal member 31 corresponds to the catalyst support mounting system. The catalytic converter 35 is interposed into an intermediate part of an exhaust pipe 37 from a car or other internal combustion engine 36 as shown in the drawing. The canning step is carried out by pushing under pressure the catalyst support 33 on which the retaining seal member 31 is wound from an open end of the casing as shown in FIG. 2.
In a catalytic converter having a construction similar to the one described above, a catalyst support mounting system has also been proposed (see Japanese Kokai No. 2002-4848) in which 0.5 to 20 wt % of an organic binder or a binder consisting of an inorganic binder is added to the mat-like matter formed by arranging an inorganic fiber in a mat form, a packing density is adjusted to be 0.1 to 0.6 g/cm3 after assembly, and when the solid proportion of the binder added to the mat-like matter is equally divided into three parts (upper part, intermediate part and lower part) in the direction of thickness and is then evaluated, the solid proportion of the binder in each of the upper and lower parts is higher than the solid proportion in the intermediate part.
Japanese Kokai No. 2000-206421 discloses a retaining seal member for a catalytic converter that uses a ceramic fiber aggregated in a mat form as a constituent element, and arranged in a gap between the catalyst support and a metal shell covering the outer circumference of the catalyst support, wherein a concavo-convex structure of an inorganic material is provided to the outer surface of the ceramic fiber. In this retaining seal member, the concavo-convex structure of the inorganic material preferably consists of metal oxide particles.